Conventional controllers detect a value of a drive current of a motor using a current detector, and feed back the detected value to a drive control system. When a value detected by a current detector of such a device is offset, problems may result when a drive current of the motor which is offset to either a positive or negative side. Potential problems include generation of an eddy current due to variation of a magnetic flux inside the rotor, reduction of magnetic power of a permanent magnet due to heat generation, reduced torque due to reduced magnetic power, increased vibration and/or noise due to torque pulsation, and so forth.
As a technique to correct an offset drive current as described above, Japanese Patent Laid-open Publication No. Hei 8-149882, for example, discloses a motor controller. This controller detects as an offset amount the value of an output current using a current detector when no drive motor is supplied to the motor, in other words, when the motor is in an inoperative state, and the drive current is corrected by the offset amount.
Another conventional controller of a motor detects a rotation angle of a rotor using an angular sensor such as a resolver, and generates a drive signal based on the detected rotation angle.
In such devices, a duty ratio of a drive signal may vary due to a detection error by the angular sensor, and the drive current may thereby be offset. The offset will be described with reference to FIG. 6.
FIG. 6(a) shows an example of output characteristics of an angular sensor. In the drawing, the abscissa corresponds to an actual rotation angle • r of a rotor (hereinafter referred to as an actual rotation angle • r), while the ordinate corresponds to a rotation angle • of a rotor as detected by an angular sensor (hereinafter referred to as detected angle •). As shown, there is a period in a single rotation cycle of a rotor in which the values detected by the angular sensor contain error. In this example, the detected angle • lags behind the actual rotation angle • r=• by an error • •.
FIGS. 6(b) and 6(c) show drive signals created based on a detected angle • by an angular sensor having the output characteristics of FIG. 6(a). FIG. 6(b) relates to a drive signal having a rectangular voltage waveform (a rectangular wave in which the level is switched at •=0, •), while FIG. 6(c) relates to a drive signal having a PWM voltage waveform. In these drawings, the upper graph relates to a drive signal at a target value Sr, while the lower graph relates to an actual drive signal Su. A higher level is represented as Hi, while a lower level of represented as Lo.
Should any error be contained in a detected angle • by an angular sensor, a duty ratio of a drive signal Su, which is created based on the detected angle, is offset from a target value Sr. In the examples of FIGS. 6(b) and 6(c), because the detected angle • is behind by an error • • from an actual rotation angle •r=•, the level of a drive signal resultantly remains at a Hi-side for a longer period than at a Lo-side, which makes the drive signal offset in a positive side.
However, because the offset of a drive current due to a detection error by an angular sensor is created while the motor is rotating, the offset cannot be corrected using an offset correction value calculated when the motor is still, as in the technique disclosed in Japanese Patent Laid-open Publication No. Hei 8-149882. Therefore, reduced motor torque, increased vibration, and/or increased noise remain likely problems.